The goal of this project is to explore the mechanisms underlying the well-recognized link between immune deficiencies and autoimmunity. We have developed a transgenic mouse model in which CD4 helper T-cells specific for ovalbumin (Ova) become tolerant (functionally unresponsive) when they encounter a secreted form of Ova (sOva) produced as a self antigen in lymphocyte-sufficient hosts. In striking contrast, if the same T-cells see the circulating Ova in a lymphopenic (RAG-/-) host, tolerance fails, resulting in a severe, systemic autoimmune reaction that is usually acutely lethal. Our hypothesis is that multiple classes of host (endogenous) lymphocytes help to maintain self-tolerance by several mechanisms, including: competition for antigen and other stimuli, antigen presentation under tolerogenic conditions, and production of regulatory cytokines. Using this model, we will address the following specific aims. 1. Regulatory functions of endogenous lymphocytes: We will determine which types of host lymphocytes function to maintain tolerance by two approaches. First, mice expressing the sOva will be depleted of selected lymphocytes (CD4 or CDS T cells, B cells, gamma-delta cells, NK-T cells). Ova-specific T-cells will be transferred into these hosts and followed for tolerance or autoimmune reactions. Second, lymphopenic hosts expressing the sOva will be reconstituted with different lymphocyte populations, and the responses of Ova-specific T cells will again be examined. 2. Mechanisms by which endogenous lymphocytes maintain tolerance: To determine how endogenous lymphocytes help to maintain self-tolerance and prevent the activation of self-reactive T-cells. We will first define the types of responses of these T-cells that correlate with tolerance vs pathologic autoimmunity. We will then inhibit or eliminate selected molecules (such as cytokines) from the self-reactive T-cells or the host, or selected non-lymphoid cell types from the host, to define how endogenous lymphocytes control the balance between tolerance and autoimmunity. 3. Imaging tolerance and autoimmunity: We will use conventional immunohistochemistry and 2-photon confocal microscopy to define the migration of self-reactive T-cells in a self antigen-expressing host, in the presence and absence of endogenous lymphocytes. These studies will tell us if the anatomy of antigen recognition and competition with host lymphocytes influence the maintenance or failure of tolerance. This project will provide valuable information about why immune deficiencies are associated with autoimmunity, and may give insights into the general mechanisms of failure of tolerance and development of systemic autoimmunity.